fix no longer working link
fix no longer working link

https://prometheus.io/docs/concepts/metric_types/ seems to make most sense.
https://github.com/OpenObservability/OpenMetrics/blob/master/specification/OpenMetrics.md looks like a more upstream definition, but it refers to types which openmetrics seems to not support currently.
adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp